1. Field of the Invention
The present invention relates generally to electronic circuits; and more particularly to operational trans-conductance amplifier circuits having an output voltage clamp circuit.
2. Description of the Related Art
The operational trans-conductance amplifier, or OTA, is a widely used circuit block in analog and mixed-signal VLSI applications. As a standalone circuit, it has an output voltage range that is determined by a combination of the device parameters, the input voltage, and the DC load current at its output. As such, it is generally not well defined. It is often desirable to limit, or clamp, the output voltage so that it does not go outside a specified range, where the range can be readily controlled.
Standard high-performance voltage clamp circuits often use a separate block to make the required comparison between the actual output voltage and a desired threshold value. This requires more area and power. While there are simpler clamp circuits that use a few elements or even a single diode, these circuits do not provide a sharp “knee” (i.e. a sharp clamping characteristic).
An example of a high-performance clamp circuit of the prior art is disclosed in Chang et al, A CMOS Differential Buffer Amplifier with Accurate Gain and Clipping Control, IEEE Journal of Solid-State Circuits, Vol. 30, No. 7, pgs. 731-735, July, 1995. Examples of the simpler circuits are disclosed in: Huang et al., A 2-V 10.7 MHz CMOS Limiting Amplifier/RSSI, IEEE Journal of Solid-State Circuits, Vol. 35, No. 10, pgs. 1474-1480, October 2000; Khorram et al., A CMOS Limiting Amplifier and Signal Strength Indicator, 1995 Symposium on VLSI Circuits Digest of Technical Papers, pgs. 95-96, 1995; Berger et al., Superintegrated Voltage Clamp, IEEE Journal of Solid-State Circuits, pgs. 231-232, June, 1973. Furthermore, U.S. Pat. No. 5,036,218 discloses a circuit (with variations) that prevents a bipolar transistor from entering saturation. It, too, uses a separately biased unit to make the comparison, though it does not operate a binary mode switch, like the circuit of Chang et al. Finally, U.S. Pat. No. 5,381,054 shows a compact multiple-input comparator circuit, but it does not clip any analog output level.
Thus, it would be thus be desirable to have an OTA circuit whose output voltage is clamped, without requiring significant additional circuit area and power, and which has a sharp knee.